Orthogonal frequency division multiplexing is a multi-carrier transmission technique that uses orthogonal subcarriers to transmit information within an available spectrum. Since the subcarriers may be orthogonal to one another, they may be spaced much more closely together within the available spectrum than, for example, the individual channels in a conventional frequency division multiplexing (“FDM”) system.
In an OFDM system, the subcarriers may be modulated with a low-rate data stream before transmission. It is advantageous to transmit a number of low-rate data streams in parallel instead of a single high-rate stream since low symbol rate schemes suffer less from intersymbol interference (“ISI”) caused by the multipath. For this reason, many modern digital communications systems are turning to the OFDM system as a modulation scheme for signals that need to survive in environments having multipath or strong interference. Many transmission standards have already adopted the OFDM system, including the IEEE 802.11a standard, the Digital Video Broadcasting Terrestrial (“DVB-T”), the Digital Audio Broadcast (“DAB”), and the Digital Television Broadcast (“T-DMB”).
Although the OFDM system is advantageous in combating intersymbol interference, it is quite sensitive to frequency deviations. The frequency deviations may be caused by the difference in the oscillator frequency of the receiver and the transmitter, or by the Doppler shift of the signal due to movement of either the receiver or the transmitter. Large frequency deviations cause significant interference between signals at different subcarriers, hence result in dramatic performance degradation. Therefore, frequency offset estimation to correct the frequency deviations is critical for delivering good transmission quality.
The existent frequency offset estimation methods include the intercarrier differential correlation (“ICDC”) method and the channel impulse response (“CIR”) based method. The ICDC method becomes cumbersome when there are many possible frequency shifts to search from since the ICDC must be computed for each frequency shift. For example, in the DAB and T-DMB standards, the number of frequency shifts is 401 in Mode I to support the maximum frequency offset of 200 kHz. The ICDC method may also return false results, due to the periodicity in the phase reference symbol, when used alone to calculate frequency offsets. The CIR based method has better performance at the cost of even greater complexity since one inverse fast Fourier transform (“IFFT”) must be performed for each frequency shift. Therefore, direct application of the existent methods to support large frequency offsets is problematic. It is therefore desirable to have novel methods for the estimation of coarse frequency offset for the demodulation of a received OFDM signal.